1. Field of the Invention
This invention relates to a flexible disk comprising a flexible casing (generally referred to as a "flexible disk jacket") and a disk-like magnetic recording medium accommodated therein for rotation.
2. Description of the Prior Art
There has been known, as a medium for magnetic recording and magnetic reproduction, a flexible disk comprising a flexible disk jacket having a central opening and a disk-like magnetic recording medium having a central opening smaller than the central opening of the flexible disk jacket in diameter. The recording medium is accommodated for rotation in the flexible disk jacket so that their central openings are substantially concentrically positioned and the portion of the recording medium around the central opening is exposed through the central opening of the jacket, thereby giving access thereto. A positioning member of a magnetic recording/reproducing system is arranged to gain access to the recording medium through the central opening of the jacket and to center the recording medium, with respect to the driving shaft of the system, which is loosely held in the jacket to be movable with respect to the jacket to a certain extent. The positioning member generally includes a collet which is inserted into the central opening of the recording medium from one side thereof to center the recording medium and is associated with a rotating member abutting against the recording medium from the other side thereof to hold the recording medium in place during recording or reproduction. However, since the recording medium is loosely held in the jacket and is movable with respect thereto, there is a possibility that the recording medium is not properly centered with respect to the positioning member to be held in a wrong position when the flexible disk is loaded in the system with the recording medium not being positioned in a predetermined position with respect to the jacket in which the center of the central opening of the recording medium is aligned with the center of the central opening of the jacket. This problem will be described in more detail with reference to FIGS. 1 to 4.
In FIGS. 1A and 1B showing a typical flexible disk, a disk-like recording medium 2 is accommodated for rotation in a square flexible jacket 1. The jacket 1 is provided with a central opening 1a and the recording medium 2 is provided with a central opening 2a. The central opening 2a of the recording medium 2 is smaller than the central opening 1a of the jacket 1 and the portion of the recording medium 2 surrounding the central opening 2a, i.e. the inner peripheral portion of the recording medium 2, is exposed through the central opening 1a of the jacket 1.
When the flexible disk is loaded in a recording/reproducing system, a rotating portion 3 of a positioning member of the system is brought into abutment against the recording medium 2 from below and a collet 4 is inserted into a circular recess 3a formed in the rotating portion 3 as shown in FIG. 2. In conventional flexible disks, a part of the inner peripheral portion surrounding the central opening 2a is apt to be sandwiched between the collet 4 and the rotating portion 3 as shown in FIG. 4 when the collet 4 is inserted into the recess 3a of the rotating portion 3 with the recording medium 2 displaced so that the central opening 2a is not concentric with the central opening 1a of the jacket 1. When a part of the inner peripheral portion is sandwiched between the collet 4 and the rotating portion 3, flatness of the recording surface of the recording medium 2 is lost and at the same time the recording medium 2 is fixed with respect to the driving mechanism of the system in an eccentric manner, thereby adversely affecting recording or reproduction.
In order to hold the recording medium 2 in a proper position as shown in FIG. 4, it is preferred that the friction coefficient between the recording medium 2 and the rotating portion 3 be as small as possible, and that the mechanical strength of the inner peripheral portion of the recording medium 2 surrounding the central opening 2a thereof be high.
In Japanese Unexamined Patent Publication No. 57(1982)-113422, it is proposed to provide the inner peripheral portion of the recording medium (to be brought into contact with the positioning member) with a protective layer containing fatty ester and polyolefin polymerizable by ultraviolet rays, thereby reducing the friction coefficient between the inner peripheral portion of the recording medium and the positioning member. In U.S. Pat. No. 4,052,750, it is proposed to bond an annular reinforcement member (resilient ring) to the inner peripheral portion of the recording medium in order to increase the mechanical strength thereof. However, the technique of the Japanese Unexamined Patent Publication does not increase the mechanical strength of the inner peripheral portion of the recording medium, though it reduces the friction coefficient between the recording medium and the positioning member of the system, while the technique of the U.S. Patent does not reduce the friction coefficient though it increases the mechanical strength.